Interpretive Summary: When peach and several other types of fruit and nut trees are replanted after a previous orchard is removed, the trees often grow slowly. Growers have learned that fumigation with methyl bromide reduces this "replant disorder" and the new trees are more vigorous and grow more uniformly. The causes of replant disorder have not been identified but are believed to oresult from a complex of major and minor soil-borne plant pests whose populations evolved with the previous orchard. We are pursuing chemical and non-chemical strategies that reduce the replant problem in peach and plum. Fallowing up to 3 years improved tree growth in most trials and reduced nematode populations. Drip-irrigation applied 1,3-D and chloropicrin combinations reduced nematodes below measureable levels to 5 ft depth and gave yields comparable to methyl bromide fumigation. Drip-applied chloropicrin alone gave the best tree growth in one trial. Fallowing can reduce the replant problem, but is costly for farmers. Drip applied fumigants can work as well as methyl bromide. These alternatives are critical for perennial crop growers that are losing one of their primary soil pest control practices.

Technical Abstract:
Methyl bromide will no longer be available for soil fumigation after 2005. It is an important pest control material for replant of orchards that suffer from "replant disorder". Several field trials tested effects of fallowing and drip-irrigation applied alternative fumigants. In all cases, methyl bromide and 1,3-D products eliminated the nematodes. Herbicide root kill and lime urea treatments did not reduce nematode counts. increasing fallow periods, especially 2 and 3 year, dramatically reduced counts. All nematodes found after 3 years of fallow were below 3 ft depth. one year of fallow increased tree growth in all four studies, although the differences in individual studies were not statistically significant. Each additional fallow years in one study resulted in improved growth in the first year, and growth after 3 yrs fallow was nearly as good as with MeBr fumigation (with no fallow). The drip-applied Telone and Telone C-35 products always gave better growth than the no-fallow checks, and growth was not different than with MeBr in most cases. These preliminary results indicate that increasing fallow periods reduce the replant disorder in most cases. Drip-applied 1,3-D and chloropicrin was effective against the replant problem.